diff --git a/ChangeLog b/ChangeLog
index 5933f06..5ef9a46 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -29,6 +29,9 @@ some minor bugs and Perl incompatibilities were fixed, including:
       existing subpattern.
   (e) A conditional recursion test such as (?(R)...) misbehaved if there was a 
       group whose name began with "R".
+  (f) The amount of memory needed for a compiled pattern was miscalculated if a 
+      lookbehind contained more than one toplevel branch and the first branch 
+      was of length zero.      
 
 One effect of the refactoring is that some error numbers and messages have 
 changed, and the pattern offset given for compiling errors is not always the
diff --git a/src/pcre2_compile.c b/src/pcre2_compile.c
index b7df32d..b120ab0 100644
--- a/src/pcre2_compile.c
+++ b/src/pcre2_compile.c
@@ -7584,11 +7584,12 @@ for (;;)
     PUT(code, 1, (int)(code - last_branch));
     bc.current_branch = last_branch = code;
     code += 1 + LINK_SIZE;
-    lookbehindlength = META_DATA(*pptr);
     }
 
-  /* Advance past the vertical bar */
+  /* Set the lookbehind length (if not in a lookbehind the value will be zero)
+  and then advance past the vertical bar. */
 
+  lookbehindlength = META_DATA(*pptr);
   pptr++;
   }
 /* Control never reaches here */
diff --git a/testdata/testinput2 b/testdata/testinput2
index ba7df01..fc16bec 100644
--- a/testdata/testinput2
+++ b/testdata/testinput2
@@ -4896,4 +4896,8 @@ a)"xI
 
 /()(?<=(?0))/
 
+/(?<!|!(?<!))/
+
+/(?<!|!|!||||||(?<!)||(?<!)!|!||(?<!)!|!(?<!)!|!|!|!||||!!|<!)!|!||||!|/
+
 # End of testinput2 
diff --git a/testdata/testoutput2 b/testdata/testoutput2
index 697cab3..50bf650 100644
--- a/testdata/testoutput2
+++ b/testdata/testoutput2
@@ -15341,6 +15341,10 @@ Subject length lower bound = 0
 /()(?<=(?0))/
 Failed: error 125 at offset 2: lookbehind assertion is not fixed length
 
+/(?<!|!(?<!))/
+
+/(?<!|!|!||||||(?<!)||(?<!)!|!||(?<!)!|!(?<!)!|!|!|!||||!!|<!)!|!||||!|/
+
 # End of testinput2 
 Error -63: PCRE2_ERROR_BADDATA (unknown error number)
 Error -62: bad serialized data